Only the owner of the database, or a superuser, can drop a
database. Dropping a database removes all objects that were
contained within the database. The destruction of a database
cannot be undone.

You cannot execute the DROP DATABASE
command while connected to the victim database. You can, however,
be connected to any other database, including the template1 database. template1 would be the only option for dropping
the last user database of a given cluster.

For convenience, there is also a shell program to drop
databases, dropdb:

dropdb dbname

(Unlike createdb, it is not the
default action to drop the database with the current user
name.)